Oxygen Isotopic Composition of U3O8 Synthesized From U Metal, Uranyl Nitrate Hydrate, and UO3 as a Signature for Nuclear Forensics

Triuranium octoxide (U3O8) is one of the main compounds in the nuclear fuel cycle. As such, identifying its processing parameters that control the oxygen isotopic composition could be developed as a new signature for nuclear forensic investigation. This study investigated the effect of different synthesis conditions such as calcination time, temperature, and cooling rates on the final δ18O values of U3O8, produced from uranium metal, uranyl nitrate hydrate, and uranium trioxide as starting materials. The results showed that δ18O of U3O8 is independent of the above-listed starting materials. δ18O values of 10 synthetic U3O8 were similar (9.35 ± 0.46‰) and did not change as a function of calcination time or calcination temperature. We showed that the cooling rate of U3O8 at the end of the synthesis process determines the final oxygen isotope composition, yielding a significant isotope effect on the order of 30‰. Experiments with two isotopically spiked 10 M HNO3, with a difference of δ18O ∼75‰, show that no memory of the starting solution oxygen isotope signature is expressed in the final U3O8 product. We suggest that the interaction with atmospheric oxygen is the main process parameter that controls the δ18O value in U3O8. The uranium mass effect, the tendency of uranium ions to preferentially incorporate 16O, is expressed during the solid–gas oxygen exchange, which occurs throughout cooling of the system.


INTRODUCTION
Nuclear forensics is essential for investigating nuclear material found outside of regulatory control or used in an act of terrorism. Tens of illicit trafficking incidents related to natural, depleted, or enriched uranium have been reported since 2019. 1 In the last three decades, nuclear forensic signatures such as elemental, isotopic, and trace element compositions of various uranium matrices were found to be valuable to understand the material history. While the rare earth element pattern and strontium and neodymium isotope ratios are unique signatures related to geographic location, 2−5 oxygen and lead are related to both geographic location and production processes. 6−17 The isotopic ratio of uranium/thorium is used to determine the material age, 18 and the elemental composition of the impurities provides information regarding the production process. 18−23 The oxygen isotopic composition is affected by various chemical and physical reactions (e.g., isotope exchange and kinetic effects), leading to preferential isotope distribution between the chemical reagents and final uranium oxide phases, and thus, it can be utilized as an additional signature for the processes involved.
The production processes of uranium fuel pellets (UO 2 ) consist of several stages, starting with milled uranium ore to produce intermediate products, mainly uranyl nitrate hydrate (UNH), ammonium diuranate (ADU), and uranium per-oxide. 24−26 Uranium oxides such as UO 3 , U 3 O 8 , and UO 2 , in the form of powders, are intermediate compounds within uranium ore processing and nuclear fuel production cycle, regardless of the above-mentioned starting materials. 24−26 U 3 O 8 is almost ever present in the production cycle of nuclear fuel. Furthermore, U 3 O 8 is a major compound in nuclear waste management due to its thermal and chemical stability. 24−28 All production processes of U 3 O 8 compounds in the nuclear industry involve three major parameters: calcination temperature, calcination time, and the cooling rate of the products. 26 Dierick et al. 12  This study focuses on the isotope signature change resulting from the manufacturing processes of U 3 O 8 under different temperatures, calcination times, cooling rates, and initial solutions. We synthesized U 3 O 8 originating from various UNH and UO 3 , via several processes commonly used in the uranium nuclear fuel cycle. We dissolved pure U metal in isotopically spiked HNO 3 to affect the uranyl ion and assess the incorporation of oxygen isotopes into U 3 O 8 .  Table 2.

MATERIALS AND METHODS
X-ray diffraction (XRD) was applied to determine the structural phase of the uranium oxides. XRD analyses (Rigaku, Ultima III) were conducted on samples weighing several milligrams under an atmosphere environment by continuous scanning at 40 kV/40 mA in the range of 10−80°at a rate of 2°/min.
2.4. Oxygen Isotope Measurement. Oxygen analysis of uranium oxides was performed with an isotope ratio gas chromatography mass spectrometer (irmGCMS, Thermo Scientific Delta Plus Advantage) and an IR CO 2 laser (10.6 μm New Wave Research25 W). The method has been previously described in detail. 15 Table 3 (starting from UNH) and in  (Table 4).         Figure 4 and Table 4. The graph shows variability (within 0.5‰ error) between 600 and 750°C, averaging 8.56 ± 0.34‰. The two samples prepared at temperatures above 750°C show higher δ 18 O values, 10.67‰ at 800°C and 9.66‰ at 850°C.
A stable δ 18 O value is achieved within the first 30 min and remains constant over calcination periods of up to 6 h and calcination temperatures between 650 and 750°C. These results point out a fast isotope exchange between solid and atmospheric O 2 . This conclusion is consistent with the fast oxygen exchange of U 3 O 8 prepared from amorphous-UO 3 under vacuum, reaching more than 94% exchange in about 23 min at 525°C, reported by Lavut et al. 30 This group also concluded that U 3 O 8 contains several types of oxygen in the lattice; however, they all exchange equivalently and have similar binding energies. A higher degree of exchange can be assumed for our experiments, as it was conducted at higher temperatures. Our data are also in agreement with Plaue et al., 11 who suggested that oxygen isotope equilibrium of U 3 O 8 with dry air was achieved in 6 h at 800°C, and a similar apparent equilibrium was measured by Klosterman et al., 16 showing oxygen isotope compositions of U 3 O 8 calcined at 700°C between 30 min and 100 h in dry air. The two U 3 O 8 , prepared at 800 and 850°C, have δ 18 O values of 10.67 and 9.66‰, respectively, which are higher than the δ 18 O values of the samples prepared at lower temperatures (Table 5 and Figure 4). A similar 18 O enrichment was reported by Klosterman et al. 16 for the difference between samples prepared at 600−700 and 800°C. We attribute this change to the preferential loss of the lighter oxygen isotope from the U 3 O 8 lattice, which starts above 750°C, 27 and to the change in cooling rates, as discussed in Section 3.4.
We tested the stability of the isotopic signal of U 3 O 8 for a longer period of calcination time, up to 168 h. Sample D-T-3, with an initial δ 18 O value of 10.16 ± 0.5‰, was calcined for 168 h and retained a δ 18 O value of 9.99 ± 0.49‰. Hence, the long-term stability of the exchange reaction can be extended to much longer periods.
3.4. Effect of the Cooling Rate of the Sample on δ 18 O Values in U 3 O 8 . The potential of fast isotope exchange during cooling as a significant factor controlling the final isotope value of U 3 O 8 is evident, due to: (1) the fast isotope exchange process between U 3 O 8 and atmospheric O 2 , (2) the lack of  correspondence with the isotope values of the starting materials, (3) the fact that all isotope data from different preparation routes converge around the same isotope value, and (4) the 2‰ enrichment of samples prepared at high temperatures. Several cooling rates, from 750°C to room temperature, were tested to determine the relationship between the cooling rate and the final δ 18 O value of U 3 O 8 ( Table 6). The routine practice was to cool samples by removing them from the furnace at the preparation temperature, cooling to room temperature over 7 min, and storing them in a desiccator under vacuum. Thus, most of the results reported here (21 samples) correspond to this cooling profile. This cooling profile yielded an average δ 18 O value of 8.15 ± 0.66‰ when the preparation temperature was set to 750°C. A faster cooling time of 2.5 min, followed by immediate transfer to an ice bath, produced a  (Table 6 and Figure 5). Applying cooling time in between the two extremes produced isotope values ranging from −22.2 to 12.3‰. Our results show that the cooling rate of U 3 O 8 can change the δ 18 O value by ∼30‰, suggesting that the isotopic quenching over the cooling process is the main factor governing the final δ 18 O value in the production of U 3 O 8 . The rapid and continuous exchange with atmospheric oxygen during cooling, to a yet unknown closure temperature, produces a wide range of isotope values. As such, it may explain the discordant isotopic values published by Plaue et al., 11 Dierick et al., 12 and Klosterman et al. 16 for α-U 3 O 8 prepared under comparable conditions but probably under different, unreported, cooling rates.   Currently, it has been possible to postulate a complete isotope exchange with atmospheric O 2 (23.5‰) at 750°C, based on the fast rate of this exchange. 37 The resetting of the oxygen isotope toward lighter isotope compositions is supported by the study of ref 39. Their calculation based on the reduced partition function ratio for uranium oxides and water shows that uraninite is depleted in 18 O with respect to the associated fluids at almost all temperature ranges (0−900°C ). As an example, exceptionally low δ 18 O values in natural uraninite, −20 to −30‰, were reported. 39 Our results clearly demonstrate such a consistent "mass effect", 38−41 when we allow the produced U 3 O 8 to cool slowly and perhaps reach an equilibrium with atmospheric O 2 at low temperatures. Oxygen fractionation in the solids depends primarily on the vibrational frequencies of the bonds with uranium in the crystal. Kinetic processes such as the diffusion of O 2 during the exchange largely depend on the solid particle size and organization, govern the rate, and point to a complex mechanism that is expressed in the final isotopic value. Such intense kinetic effects during cooling undermine the possibility of providing direct forensic geolocation information. However, our study highlights additional factors that control the fabrication process and expands the possible identification and characterization of nuclear production plants. Practically, this additional factor, the cooling time at the end of the production stage of U 3 O 8 , can be collected from different nuclear production plants and added to a worldwide nuclear forensics database.

CONCLUSIONS
This study examined the effect of different starting materials, synthesis conditions such as calcination time, temperature, and cooling rate on the final δ 18 O values of U 3 O 8 . The average δ 18 O values of U 3 O 8 synthesized from UO 3 and UNH are 8.94 ± 0.50‰ and 8.15 ± 0.66‰, respectively. The similarity of the δ 18 O values of U 3 O 8 obtained for both preparation routes emphasizes that a common external source determines the oxygen isotopic composition. The similar δ 18 O values obtained for U 3 O 8 synthesized from different isotopically spiked nitric acids imply that the final isotopic composition of U 3 O 8 is independent of the starting material isotopic composition and further suggest the involvement of another oxygen source during calcination. Our kinetic experiments show that within 30 min, a stable δ 18 O value is achieved and remains stable over calcination times of 0.5 to 168 h and at calcination temperatures between 650 and 750°C. It suggests that a fast oxygen isotope exchange occurs in this system. The effect of cooling profiles on the oxygen isotopic composition is determined by changing the cooling rate from 750°C to room temperature, between 2.5 min to 33 h. Our results show that the cooling rate of U 3 O 8 changes the final δ 18 O value by ∼30‰, suggesting that the cooling profile is the main factor governing the final δ 18 O value in U 3 O 8 in the production process. The resetting of the oxygen isotope toward lighter compositions can be explained by the uranium mass effect. This study contributes to the development of a new signature to be used in nuclear forensic investigations.